Windows 10 phonesHere are the official details from the customary blog post that was supposed to go up at the time of first Windows 10 for Phones preview release.

Windows 10 for Phone demo video on a (hmmm) Lumia 1520:

Windows 10 for Phone first preview changelog:

Much of the cool stuff will come with future builds, yet here are the official details for the latest Build.

  • Full-size Background Image for Start: We believe phones should be intensely personal – so we’ve added another option to customize the start screen with a full-size background image.
  • More Quick Actions in Action Center: We’ve increased the number of quick actions available to you. Windows Phone 8.1 has four programmable quick actions – but with Windows 10 you also get an expanded view that can now have up to three rows.
  • Interactive Notifications: Notifications are now interactive, allowing you to take action directly like dismissing an alarm, or seeing images for maps. For example, for text messages, when the toast pops, you can quickly reply inline via text or voice.
  • Significantly enhanced speech-to-text capability: You can talk to virtually any data field you choose. Your words show up as you speak them – and punctuation appears automatically.  This feature is smart enough to understand when to use ‘two’ – the number — instead of ‘too’ as in ‘also’.
  • More powerful Photos app: From your very first launch, Photos will show the aggregated set of all your local photos and all your OneDrive photos

What to expect in Future Builds:

There are also some things Joe showed that will be coming in future builds.  For example, our Office team has built some great features into new universal apps for Word, Excel and PowerPoint, which work across phones, tablets, and PCs running Windows 10.  Although they are not included in this build, you will see them coming soon.  In upcoming builds we’ll also include the new Mail and Calendar Universal Apps from the Outlook team in Office, as well as IP-based messaging from Skype and others, integrated directly into Messaging experience.

or Better read at our dedicated Windows 10 page

Known Issues & Workarounds:

Existing alarms will not be migrated to Windows 10Reset alarms after upgrade
Wi-Fi settings don’t roam when phone is upgraded to Windows 10Manually set Wi-Fi settings when on Windows 10
VPN not available in current Windows 10 builds. Setting will fail to launch. Coming in a future update
Additional language keyboards may not be present and may not be able to install on US builds after upgrading to Windows 10Reopen the Windows Insider app and re-select Insider Fast/Slow again. The download of keyboard packages will then work
Photos app will fail to launch periodically when attempting to add a photo attachment to an email, OneNote or Facebook item.Try again. Succeeds after a few attempts.
Quiet Hours is missing the automatic rule that enables Quiet hours during calendar appointments marked busy. Also the setting “Anyone breaks through if they call 2 times in 3 minutes” us unchecked but enabled.
Apps are not displayed in Battery saver after the upgrade.
Access Point Names (APNs) in recovery images can fall out of date. For some phones/networks, this will block the cellular data and MMS capabilities of a phone when recovering back to the base image.Before using the Windows Mobile Recovery Tool to recover, record the specific APN settings on your phone.  Set manually once phone restored
Cellular data not working while roaming, despite enabling data roamingThe setting doesn’t work the first time it’s enabled. Turning off and then turning on again will enable data while roaming
Using a Bluetooth headset while playing back video results in low frame rate for video, due to a bug in the interaction between video playback and Bluetooth.Use wired headset for listening to audio when playing videos.
Sync to Microsoft Band doesn’t work after updating to Windows 10. If you have your phone synced to Microsoft band, it will not pair after the update to Windows 10.After upgrade to Windows 10, go to Bluetooth settings, un-pair the Microsoft Band device and re-pair it.
DataSense resets data use history on upgrade to Windows 10.No workaround. Will calculate correctly after the reset.
Trying to set lock screen picture will fail at timesRe-attempt the operation.
After upgrade, tapping on “Playlist” from start screen or from within Music App fails to play music. Playlists are greyed out.
Cortana tile and settings are missing on the start screen after upgrade

Gabe Aul has also clarified about only selecting Lumia 630 family, Lumia 730, Lumia 830 for the first preview build.

Support for more phones will be phased in with each new build, and we’ll announce which devices are being added when the build comes out. Expect the list to grow slowly at first but expand comprehensively over time. For this first preview build these are the devices supported:

Lumia 630Lumia 638
Lumia 635Lumia 730
Lumia 636Lumia 830

For these devices we support configurations from almost all mobile operators around the world, the most notable exception is China which isn’t yet supported for the first preview.

Some context on why we chose these and not higher end phones like the 930/Icon or 1520: We have a feature that will be coming soon called “partition stitching” which will allow us to adjust the OS partition dynamically to create room for the install process to be able to update the OS in-place. Until this comes in, we needed devices which were configured by mobile operators with sufficiently sized OS partitions to allow the in-place upgrade, and many of the bigger phones have very tight OS partitions.

Read more here